1. Field Of The Invention
The present invention relates to a vacuum roll apparatus for transferring a web supported on a dryer felt from a first to a second drying cylinder of a dryer section. More specifically, the present invention relates to a vacuum roll apparatus for a dryer section in which the dryer felt is disposed between the web and the vacuum roll apparatus during movement thereof past the vacuum roll apparatus.
2. Information Disclosure Statement
With the introduction by Beloit Corporation of the Bel-Champ.TM. type dryer section, drying speeds have been greatly increased. Additionally, the quality of the resultant sheet has improved, particularly due to an enhanced cross-machine directional restraint of the web during movement thereof through the dryer section. Bel-Champ is a common law trademark of Beloit Corporation.
However, with the Bel-Champ.TM. type dryer section as disclosed in U.S. Pat. No. 4,934,067, a vacuum roll is disposed between each adjacent drying cylinder.
Such vacuum rolls typically use cross-machine directional internal seals or glands to isolate vacuum substantially within the region of the roll covered by the paper and dryer fabric. The provision of such seals involves considerable expenditure in terms of initial capital outlay and maintenance costs.
In an attempt to reduce the aforementioned costs, field trials have been carried out on vacuum rolls in which the internal seals were removed so that a vacuum be applied to a cavity extending along the length of a perforate roll shell.
Generally, these trials suggested that above speeds of 1,000 feet per minute, insufficient vacuum is available at the roll surface, even when relatively high vacuum levels are applied to the roll to hold the web onto the dryer felt.
One explanation of the aforementioned problem is that due to the high rotational speed of the roll shell, laminarization or retransition from turbulent to laminar flow takes place. During rotation at high speeds, viscous effects cannot be neglected near the roll surface. Formation of closed vortices at the center of the roll become a real possibility. Such a phenomena is addressed in an article entitled "Topics In Applied Physics", Volume 45, under the heading "Hydrodynamic Instabilities and The Transition to Turbulence" by H. L. Swinney and J. P. Gollub.
Additionally, at the exit region of the roll shell, that is from the ends of the roll shell and the stationary pipe connected to a vacuum source, sudden changes in rotational speed causes the flow to become unstable and the development of air flow is delayed.
The present invention overcomes the viscous effects near the internal surface of the roll shell and disrupts closed vortices at the center of the roll shell by means of fins attached to the inside of the vacuum roll.
Additionally, in order to overcome the instability of the flow from the vacuum roll to the pipe connecting the roll to the vacuum source, flow deflectors are installed, such deflectors rotating with the roll shell.
Therefore, it is a primary objective of the present invention to provide a vacuum roll apparatus which overcomes the aforementioned problems associates with the prior art arrangements and which makes a considerable contribution to the art of drying a web of paper.
Another object of the present invention is the provision of a vacuum roll apparatus which overcomes the problem of laminarization or retransition from turbulent to laminar flow.
Another object of the present invention is the provision of a vacuum roll apparatus which overcomes the problem of viscous effects at high rotational speeds of the vacuum roll apparatus and close vortices at the center of the roll.
Another object of the present invention is the provision of a vacuum roll apparatus which requires less capital outlay and maintenance costs relative to prior art vacuum rolls.
Another object of the present invention is the provision of a vacuum roll apparatus which does not require internal seals for contacting the inner surface of a roll shell, thereby reducing the costs involved in machining the internal surface of the vacuum roll to demanding tolerances.
Another object of the present invention is the provision of a vacuum roll apparatus in which the maintenance costs thereof are reduced due to the elimination of wearing contact of the surface of and seal relative to the internal surface of the roll shell.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter, taken in conjunction with the annexed drawings.